1. Field of the Invention
The invention relates to an image signal processing apparatus for compressing image data such as a photographing output of a video camera of a handy type or the like, an image signal recording apparatus for recording compression image data to a recording medium, and an image signal reproducing apparatus for reproducing the compression image data from the recording medium and, more particularly, to a correction of a camera shake of a video camera.
2. Description of the Related Art
When photographing by using a video camera of a handy type, there is a problem such that a reproduction picture plane is shaken due to a camera shake. To solve the problem, there is considered a method whereby a motion vector is detected and image data stored in an image memory is corrected on the basis of the motion vector. The detection of the motion vector is performed by, for example, a block matching. That is, the picture plane is divided into a number of areas (referred to as blocks), an absolute value of a frame difference between a representative point of a previous frame which is located at the center of each block and pixel data in the block of the present frame is arithmetically operated and the absolute value of the frame difference is integrated with respect to one picture plane, thereby detecting the motion vector of the whole picture plane from the position of the minimum value of integration frame difference data obtained. The detected motion vector is converted into a correction signal and a correction to move an original image is performed by the correction signal.
For example, in FIG. 1A, a picture frame La shown by a broken line is a picked-up image and its position is corrected to a picture frame Lb shown by a broken line by a camera shake correction. In the image in the picture frame Lb after completion of the correction, since there is no pickup image in the area shown by a hatched portion, a drop-out of images occurs. As one method of solving the problem, there is a method of enlarging the picture frame to some degree as shown by Lc in FIG. 1B. Thus, it is possible to prevent the drop-out of images.
However, the enlargement of the image makes a reading speed of the image memory slower than a writing speed of the memory and is performed by a process of interpolating lacking pixel data. Therefore, a resolution of the enlarged image deteriorates as compared with that of the original image. Thus, there is a problem such that a picture quality after completion of the camera shake correction is not good.
Other problems in the conventional camera shake correction will now be described hereinbelow. This is a problem such that a motion of a large area in the picture plane is erroneously discriminated as a camera shake. When using a method of dividing one picture plane into (4.times.4=16) macroblocks and detecting a motion vector in each macroblock, for example, as shown in FIG. 2A, when a person moves in the direction from the right to the left in the picture plane, the motion vector shown by an arrow is detected in each block. The motion vectors in the other blocks in which no person (edge) is included are detected as 0.
Hitherto, the motion vector of each macroblock is discriminated by using a decision by the majority and the motion vectors as major vectors are used as motion vectors for the camera shake correction. As in the conventional apparatus mentioned above, however, when an object having a large area exists at the center of the picture plane and the object moves, there is a possibility such that the movement of the object is erroneously discriminated as a camera shake. It is, consequently, necessary to avoid such an error discrimination.
Further, there is also a case where a plurality of motion vectors exist as motion vectors of the macroblocks. For example, as shown in FIG. 2B, the motion vectors corresponding to the motion which occurred due to the camera shake and motion vectors which occurred due to the motion of the object in the picture plane are synthesized, so that a plurality of motion vectors are detected. Like an example of FIG. 2B, there is a problem such that in a state in which there is a motion of the object having a large area, when the majority decision is performed as in the conventional apparatus, the motion vector due to the camera shake cannot be correctly detected.
Further, in an image signal recording apparatus such as a VTR of a video camera integrated type, it is general to record a signal which was subjected to a camera shake correction as an image signal to be recorded. Thus, however, the photographed image itself cannot be reproduced and, when the camera shake is not preferably corrected, there is a problem such that the correction is impossible on the reproducing side.
As mentioned above, the conventional method of detecting the motion vector and correcting a camera shake on the basis of the detected motion vectors has various problems. Further, in addition to the above-mentioned problems, there is a problem such that a hardware to detect the motion vectors is necessary and a scale of the hardware for the camera shake correction increases.